Atoms vs. Ions: Intermediates in Reversible Electrochemical Hydrogen Evolution Reaction

We present a critical analysis of the mechanism of reversible hydrogen evolution reaction based on thermodynamics of hydrogen processes considering atomic and ionic species as intermediates. Clear distinction between molecular hydrogen evolution/oxidation (H2ER and H2OR) and atomic hydrogen evolution/oxidation (HER and HOR) reactions is made. It is suggested that the main reaction describing reversible H2ER and H2OR in acidic and basic solutions is: H3O++2e−⇌(H2+)adH2+OH− and its standard potential is E0 = −0.413 V (vs. standard hydrogen electrode, SHE). We analyse experimentally reported data with models which provide a quantitative match (R.J.Kriek et al., Electrochem. Sci. Adv. e2100041 (2021)). Presented analysis implies that reversible H2 evolution is a two-electron transfer process which proceeds via the stage of adsorbed hydrogen molecular ion H2+ as intermediate, rather than Had as postulated in the Volmer-Heyrovsky-Tafel mechanism. We demonstrate that in theory, two slopes of potential vs. lg(current) plots are feasible in the discussed reversible region of H2 evolution: 2.3RT/F≈60 mV and 2.3RT/2F≈30 mV, which is corroborated by the results of electrocatalytic hydrogen evolution studies reported in the literature. Upon transition to irreversible H2ER, slowdown of H2+ formation in the first electron transfer stage manifests, and the slope increases to 2.3RT/0.5F≈120 mV; R,F,T are the universal gas, Faraday constants and absolute temperature, respectively.